Planar displays are popular for portable displays and displays with space limits because they are light and small in size. To date, planar displays in addition to liquid crystal displays (LCD), organic electro-luminescent displays (OLED), plasma display panels (PDP) and so on, as well as a mode of the optical interference display are of interest.
U.S. Pat. No. 5,835,255 discloses an array of display units of visible light that can be used in a planar display. Reference is made to FIG. 1, which depicts a top view of a light interference display unit disclosed in the prior art. A plurality of first electrodes 102 is located in parallel on a substrate 100. A plurality of the second electrodes 104 is located in parallel on the first electrodes 102 and is arranged vertically with the first electrodes 102. A plurality of posts 106 is located between the first electrode 102 and the second electrode 104, and a cavity (not shown) is subsequently formed. Reference is made to FIG. 2, which depicts a cross-sectional view according to a cutting plane line I–I′ in FIG. 1. Every optical interference display unit 108 comprises two electrodes, 102 and 104. Posts 106 support these two electrodes 102 and 104, and a cavity 110 is subsequently formed. The distance between these two electrodes 102 and 104, that is, the length of the cavity 110, is D. One of the electrodes 102 and 104 is a semi-transmissible/semi-reflective layer with an absorption rate that partially absorbs visible light, and the other is a light reflective layer that is deformable when voltage is applied. When incident light passes through the electrode 102 or 104 and arrives in the cavity 110, in all visible light spectra, only visible light with wavelength corresponding to formula 1.1 can generate a constructive interference and can be emitted, that is,2D=Nλ  (1.1)
where N is a natural number.
When the length D of cavity 110 is equal to half the wavelength times any natural number, a constructive interference is generated and a sharp light wave is emitted. In the meantime, if the observer follows the direction of the incident light, a reflected light with wavelength λ1 can be observed. Therefore, the display unit 108 is “on”.
One of the first electrode 102 and the second electrode 104 is a deformable electrode or a movable electrode. It shifts up and down by applying a voltage. While driven by the voltage, the deformable or movable electrode is deformed and falls down towards another electrode due to the attraction of static electricity. At this time, the distance of the length of the cavity 110 changes. All incident light in the visible light spectrum is filtered out and an observer who follows the direction of the incident light cannot observe any reflected light in the visible light spectrum. The display unit 108 is now “off”.
Referring again to FIG. 1, besides the post 106, support structure 112 is located between two second electrodes 104 to support the second electrode 104. Without the support structure 112, the edge of the second electrode 104 sags down due to a lack of support. Therefore, the length of the cavity 110 is not uniform. For the display unit 108, non-uniformity of the length of the cavity 110 results in reflected light with at least two different wave-lengths; therefore, the resolution of the reflected light becomes worse and the display unit may display more than one color.
Reference is made to FIG. 3A, which depicts a cross-sectional view according a cutting plane line II–II′ in FIG. 1. The method for forming the structure illustrated in FIG. 3A is depicted in FIG. 3B. A transparent conductive layer, a absorption layer and a dielectric layer (all not shown) are formed sequentially on a transparent substrate 100. The transparent conductive layer, the absorption layer and the dielectric layer form a first electrode 102. A sacrificial layer 114 is then formed on the first electrode 104. The material for forming the dielectric layer comprises silicon oxide and silicon nitride; the material for forming the transparent conductive layer comprises indium tin oxide, indium zinc oxide and indium oxide; and the material for forming the absorption layer is metal. Next, a lithography process and an etching process are performed to form an opening 116 in the sacrificial layer 114 and the first electrode 102. A photoresist layer is spin-coated on the sacrificial layer 114 and fills the opening 116. An exposure process is performed on the photoresist layer and a support structure 112 is formed in the opening 116.
A conductive layer 118 is formed on the support structure 112 and sacrificial layer 114. A spin-coating process and a lithographic process are performed sequentially to form a patterned photoresist layer 120 on the conductive layer 118. An opening in the patterned photoresist layer 120 exposes the underlying conductive layer 118 located on the support structure 112. The patterned photoresist layer 120 is used as an etching mask to remove the exposed conductive layer 118; then, the second electrode 104 settled in parallel with the first electrode 102 illustrated in FIG. 1 is formed. Finally, the photoresist layer 120 is removed and the optical interference display unit 108 is formed.
Generally, a material used to form the support structure 112 is photoresist; therefore, the support structure 112 is always damaged or removed completely in the step of removing the photoresist layer 120 and a structure illustrated in FIG. 3C is formed. Reference is made to FIG. 3C, which depicts cross-sectional views of an optical interference display unit which lack the support structure. Because the support structure is damaged or removed, the edge of the second electrode 104 gets no support and sags in a direction indicated by arrow 105. The length of the cavity 110 is not uniform because of the sagging edge of the electrode 104. Therefore, the disadvantages of a worse resolution and wrong color of the optical interference display unit can't be avoided.
Therefore, it is an important subject to provide a simple method of manufacturing an optical interference display unit structure, for manufacturing a color optical interference display with high resolution, high brightness, simple process and high yield.